Here's Who To Blame For Half-Life 2: Episode 3's Long GestationHalf-Life 2's "episodic" expansions are hardly what you'd call episodic in the gaming sense of the word. Other series can knock out two, sometimes even three full games in the time it takes Valve to cobble together a single 4-6 hour mission pack. Of course, the wait's always worth it, since nobody does it quite like Valve does it, but still. Long waits hurt. And when something hurts, we like to blame somebody. So who can we blame for the agonising wait between Episode 2 and Episode 3? Blame Left 4 Dead. And blame Team Fortress 2. Valve's Doug Lombardi:
So I mean, we're taking more time because it's a more ambitious project. We also sort of had Left 4 Dead become this big thing in the middle of it that we wanted to spend more time on, and you've just got a lot of stuff that we spent this year doing with Team Fortress 2 updates and what have you. So there's just a lot going on, and then an ambitious project on top of it caused the schedule to move out a little bit from the delta of the last three releases in the franchise.
Goddamnit Team Fortress 2 and Left 4 Dead, how dare you... ah, who are we kidding. We can't get mad at you guys.

Valve on Episode Three: 'A More Ambitious Project' [Shacknews]